The invention relates to the use of specific catalysts for the production of polyisocyanate polyaddition products, in particular for the coatings sector.
Polyurethane coatings have been known for a long time and are used in many fields. They are generally produced from a polyisocyanate component and a hydroxyl component by mixing immediately prior to application (2K technology). For light-fast coatings, polyisocyanate components based on aliphatic polyisocyanates are generally used, which enter into a reaction with the hydroxyl component markedly more slowly as compared with products having aromatically bonded isocyanate groups. In most cases, therefore, the reaction must be catalysed. Heating is additionally carried out, where possible, in order to accelerate the reaction further. Organic tin compounds, in particular dibutyltin dilaurate (DBTL), have proved to be suitable as catalysts. They have the general disadvantage of an unfavourable ecological profile, which has already led, inter alia, to the complete removal of the substance class of the organotin compounds from marine paints, to which they were added as a biocide.
A general disadvantage of 2K technology is that the NCO—OH reaction proceeds slowly even at room temperature and markedly more quickly when catalysed, with the result that only a very narrow time window (open time) is available for the processing of the ready formulated mixture of such a 2K system, which window is further shortened by the presence of the catalyst.
There has therefore been no lack of attempts to develop catalysts which scarcely accelerate the crosslinking reaction on preparation of the 2K mixture but accelerate it significantly after application (latent catalysts).
A class of latent catalysts which is used in particular in the cast elastomers sector is organomercury compounds. The most prominent representative is phenyl mercury neodecanoate (trade names: Thorcat 535, Cocure 44). However, inter alia because of the toxicology of the mercury compounds, these catalysts do not play any role in coatings technology.
The focus in this sector has tended to be on systems which can be activated chemically, for example by (atmospheric) moisture and/or oxygen (see WO 2007/075561, Organometallics 1994 (13) 1034-1038, DE 69521682), and photochemically (see U.S. Pat. No. 4,549,945).
Disadvantages of the last-mentioned two systems of the prior art are on the one hand that it is difficult to ensure a defined, reproducible migration of (atmospheric) moisture or oxygen independently of the coating recipe (degree of crosslinking, glass transition temperature, solvent content, etc.) and of the ambient conditions and on the other hand that, in particular in the case of pigmented systems, there are limits to the use of radiation sources for activating the photolatent catalyst.